Crustal Emission and the Quiescent Spectrum of the Neutron Star in KS 1731−260

(Abridged). The type-I X-ray bursting low mass X-ray binary KS 1731-260 wasrecently detected for the first time in quiescence by Wijnands et al.,following an approximately 13 yr outburst which ended in Feb 2001. Unlike allother known transient neutron stars, the duration of this recent outburst is aslong as the thermal diffusion time of the crust. The large amount of heatdeposited by reactions in the crust will have heated the crust to temperaturesmuch higher than the equilibrium core temperature. As a result, the thermalluminosity currently observed from the neutron star is dominated not by thecore, but by the crust. Moreover, the level and the time evolution of quiescentluminosity is determined mostly by the amount of heat deposited in the crustduring the most recent outburst. Using estimates of the outburst mass accretionrate, our calculations of the quiescent flux immediately following the end ofthe outburst agree with the observed quiescent flux to within a factor of afew. We present simulations of the evolution of the quiescent lightcurve fordifferent scenarios of the crust microphysics, and demonstrate that monitoringobservations (with currently flying instruments) spanning from 1--30 yr canmeasure the crust cooling timescale and the total amount of heat stored in thecrust. These quantities have not been directly measured for any neutron star.Comment: Submitted to ApJ; 7 text pages, 3 figures, uses emulateapj.sty and apjfonts.st